Moving from Nvidia to Ati... do i need to format?

I have recently upgraded from 8600GTS (Nvidia) to 4870 (ATI), and i have been getting game crashes, blue screens, and many short game freezes (when the game stops for about 1-2 secs and continues... but every 10 seconds)

i have uninstalled the NVidia drivers before removing the Nvidia card.
then i replaced the cards, and installed ATI drivers (Catalyst 8.12)
after getting problems, i used driver cleaner pro in safemode, and it did delete some nvidia DLLs, but it didnt help much, it fixed some stuff though.

on the desktop, graphical stuff seem to load slower.
in games, there are a lot of small freezes, (in CoD4, the FPS counter shows the avarage FPS, and the minimum/maximum time between the frames in that second. on low quallity on the 8600gts i had 125fps and time between frames was around 5-11 to 7-9 (125 is 8ms). on the 4870 its 125, with 7-9 most of the time, but every 3-4 seconds it drops to 5-25, meanning it froze for 25ms).

some of the games i tried:
TrackManiaForever gave me a BSOD on highest quallity (twice), on low quallity it works fine though.
Crysis crashed after loading a level (with a "dont send" message).
Crysis Warhead's first level worked, then crashed with the same message after loading the 2nd level.
Fallout 3 was the only playable game, but it also had freezes.
3Dmark06 said my score is 12,780, and comparing to 15 other Systems with same hardware i was way under (Avarage was 14,500 i think).

also, the card wasnt as fast as i excpected it to be....

in short, my question is, do i need to format and reinstall windows or not?

My System:
E8400 3.00GHz (Usally with 3.6 OC, but removed it before installing the new card).
ASUS 4870 512MB.
Brand new ThermalTake 600W power supply (dont tell me its a power problem).
Windows XP Pro 32bit.